The present invention relates generally to computer measurement systems. More specifically, the present invention relates to the measurement of features on photographic masks used in semiconductor manufacturing.
The recent introduction of advanced sub-micron sized semiconductor devices require reduced critical dimensions and increased packing densities. At these sub-micron sizes and high densities, even defects and imperfections as small as 1 micron and below are problematic and need to be detected and evaluated. Imperfections in the reticle generated by a photographic (xe2x80x9cphotomaskxe2x80x9d) mask manufacturing process are one source of defects. Errors generated by such a photomask manufacturing process have become an important issue in the manufacture of semiconductor devices at these sub-micron sizes. Defect inspection techniques for masks are therefore becoming to play a more important role in mask making and quality assurance.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired, and prevents the needless and costly hold up of masks that do meet specification. However, one of the problems of assessing reticle quality at these sub-micron levels on an automatic inspection system is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment. For example, as the line width on sub-micron masks approaches 0.1 micron, the ability to measure feature sizes at 1 micron and below becomes very important. Current production machines have an accuracy of 0.1 micron to 0.2 micron, but this is not sufficient.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value. Consequently, many mask makers have incorporated measurement aids at the inspection station or have moved the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to xe2x80x9ceyeballxe2x80x9d the boundaries of the feature. This activity is very subjective and can lead to an error in the measurement of the feature.
For example, particle size is conventionally measured by measuring the distance between opposite edges of the particle. Once a feature is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the feature and another cursor on the other side of the feature. The operator must judge for himself the exact boundaries of the feature and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the diameter of the feature. This technique is not optimal.
Firstly, this measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the feature. The operator may misjudge the type of a feature, its boundaries, or may simply misplace a cursor even if the feature is visible. The software then blindly calculates the distance between the cursors, without regard for the type of feature, its true boundaries, etc. The above technique may be performed with a standard video microscope and has an accuracy of about 0.1 micron, but is completely subject to the operator""s skill level and interpretation.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these smaller and smaller feature sizes. Current techniques do not adequately address the non-linearities associated with such measurements.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the feature, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure feature sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
Therefore, an objective feature measurement tool is desirable for use with a photomask inspection tool that can provide reliable and repeatable measurements of features of less than about one to two times the microscope resolution (or about less than one micron for optical microscopes). It would be especially desirably for such a tool to operate in a fast and highly practical manner in a production environment. More specifically, it would be desirable to be able to determine the radius of curvature of a corner of a line, especially at sizes that approach, or are less than, the wavelength of light or the particle beam being used where blurring is a problem.
The present invention discloses a measurement tool that provides an objective, practical and fast method for accurate sizing of mask features found with an automatic inspection tool (such as a video inspection machine). Dimensions can be measured by using gray scale image information provided by the automatic inspection tool. The present invention may be used while the photomask is in-place at the inspection station, and there is no need for the mask to be removed to a different machine for measurement. The dimension of the feature is then automatically identified and measured quickly by the measurement tool of the present invention.
Benefits include avoiding repairing masks within specification, and equivalent results whether measured by customer or supplier (when calibrated with the same reference). Operator productivity and tool utilization is improved by rapid measurements taking place at the inspection station.
The disclosed measurement tool objectively and repeatedly measures the radius of curvature of mask features for characterizing photomask quality. The measurement tool operates automatically and is not dependent upon operator judgment. Corner radii from 0.1 to 1.0 microns can be measured, repeatable to 0.02 microns and accurate to 0.05 microns with a typical SEM calibration. Additionally, the measurement tool provides automatic measurements in 1 to 5 seconds (including operator actions).
The disclosure provides a variety of techniques useful for implementing the present invention. In one technique, multiple regions of interest are formed surrounding a feature and an intensity profile is developed for each region of interest. A total light flux measurement is calculated for each profile, and one of the light flux measurements is chosen as the best flux value. A good quality profile is chosen such that the total flux measured from the profile is proportional to the area of the feature. Multiple regions allow for angled lines. A region of interest surrounds the feature and a profile for the feature is produced by summing columns of pixels across the feature site in the region of interest. A baseline intensity value is determined for the profile and is subtracted from the profile in order to determine the total flux passing through the feature. Subtraction of a baseline removes background intensities and obviates the need to obtain a reference image.
In the specific embodiment disclosed, the present invention is able to accurately determine the radius of curvature of corners of features on a variety of media, and especially at subresolution sizes.
Thus, by providing an extremely accurate measurement of mask features, the disclosed measurement tool helps to avoid unnecessary mask repairs and allows for improved process control. Also, operator variability is eliminated, and overall productivity and mask throughput is increased due to the accurate measurements in-place and documentation produced in seconds. Because the measurements are automatic, operator training is minimal.
The present invention is also able to measure specific dimensions of features that are under about one to two times the microscope resolution. Features can all be accurately sized at this subresolution size even though blurring is present. The prior art has had difficulties in measuring the dimension of a feature at subresolution sizes. Frequent reference is made herein to the applicability of the invention for sizes of features less than about 1 micron; this range applies to visible light, where the wavelength is about 0.5 micron. For other wavelengths, the present invention is generally suitable for features having sizes less than about two times the blur distance of the optics being used, or for features that are less than about twice the wavelength being used. The invention is especially suited for features having sizes close to or less than the wavelength being used.